If this is the first time I'm playing Death From Below this game, so there's none in the trash, and I kill a 3 might unit, can I recast this same card, or do I need to have already played a different Death From Below before so that there would be one in the trash already? In other words, when it resolves, does it count as being in the trash for replay purposes? Basically, can I recast the first Death From Below or only starting with the second Death From Below?

To answer your question: Yes, you can recast the same copy of Death from Below, but only after it has finished resolving and moved to your trash. You cannot play the card while it is currently resolving on the chain.

The Timing and Logic

The interaction follows the standard rules regarding card zones and resolution:

1. Card Location: According to Rule 351.2 and FAQ #8386, a spell is not moved to the trash until after its game effects have been fully executed and it has finished resolving.
2. Instruction Evaluation: Per FAQ #9400 and FAQ #9402, when Death from Below resolves, the game evaluates its instructions from top to bottom. During the resolution of its final instruction ("you may play this from your trash"), the card is still on the chain.
3. The Conflict: Because the card is still on the chain—not in your trash—while its own effect is being evaluated, the instruction to play it from the trash cannot target the card that is currently resolving.

Sequence of Events

• Play: You play your copy of Death from Below from your hand. It enters the chain.
• Resolution:
  • The "Kill a unit" effect executes.
  • The game evaluates: "If it had 3 [S] or less, you may play this from your trash."
  • At this moment, Death from Below is a Finalized Chain Item, not in your trash.
• Cleanup: After the spell finishes resolving, it moves to the trash (Rule 359.3.d, FAQ #8386).
• Trigger: Per FAQ #9402, the "play this from your trash" instruction functions as a trigger that allows you to play the card from the trash after the current resolution has concluded.

Conclusion: You cannot recast the card while it is currently resolving. However, because the effect creates a trigger that occurs after the card hits the trash, you can use that same copy to pay its cost and play it from your trash once it has fully moved there.

Note: I could not fully verify this ruling against the retrieved rules.